1. Field of the Invention
The present invention relates to a picture processing apparatus, more particularly, to a picture processing apparatus for correcting a picture containing a document image with a spatial curvature, such as books or the like, into a distortion-free picture.
2. Description of the Related Art
As one prior art, picture distortion correction apparatus is disclosed in, for example, Japanese Unexamined Patent Publication JP-A 6-164852(1994), which apparatus is such constituted that a document guide having a black surface is provided at a document table, for detecting the boundary between a document image and a document table image , to calculate a magnification/reduction ratio of the picture on the basis of detection results to correct picture distortion.
Another prior art apparatus is disclosed in Japanese Unexamined Patent Publication JP-A 181828 (1994), which is a picture synthesizer apparatus for correcting picture distortion, wherein a similar stripe is added to the above mentioned document guide so as to enhance document shape recognizability by varying the luminances of such stripe pattern and the surface of the document.
Still another prior art is found in Japanese Unexamined Patent Publication JP-A 5-161004(1993), which is designed to correct picture distortion and luminance variations based on the boundary between a document and a document table.
However, the prior art disclosed in Japanese Unexamined Patent Publication JP-A 6-164852 (1994) is merely drawn to line inclination/curvature correction (correction of expansion distortion in the longitudinal direction of a document) and is silent about correction of distortion due to document inclination and rotation. This prior art is encountered with another problem in that page bundling seam line-included gutter marginal portions of spread pages of a book would act as a shadow area and can thus be printed in black due to lack of luminance correction schemes.
The above described three prior art approaches suffer from a problem which follows: Where a document is placed so as to be inclined or rotated, lower part of the document can no longer be corrected accurately during correction of distortion-enhanced document portions in the vicinity of page bundling seam line of a book because of the fact that these approaches take no account of skew such as document rotation, and designed to perform corrections merely based on an edge from the boundary of a document upper section assuming the document height to be identical in a direction perpendicular to its seam line.
A further problem faced with the prior art is that even where a document to be scanned is disposed along the document guide, document skew usually takes place, which rendering it difficult to successful correct the distortion in the vicinity of a seam line of a book.